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Enzymatic hydrolysis of cellulosic biomass is a key step of biological routes for production of fuels and chemicals. Economic considerations for large-scale implementation of the process demand operation in the high solids regime. In this regime , the biomass is processed at concentrations of 15% or higher by weight , forming a high viscosity slurry which introduces processing challenges especially in the initial stages of hydrolysis (liquefaction). Rheological measurements with rotational rheometers are challenging in these materials due to rapid changes in rheology that require high time resolution , and the large particle sizes which cause gap effects and settling. We employed a non-invasive real-time rheometer (FlowScan(TM) , Aspect Imaging) to characterize the real-time evolution of yield stress of cellulose undergoing enzymatic hydrolysis in a recycle-flow reactor. The hydrolysis was performed in fed-batch mode , with fibers being added at various time points. The suspension exhibited sustained changes in rheology upon initial dispersion of fibers in water (prior to the first addition of enzyme) , which was interpreted as hydration or swelling. Each addition of fibers caused a rapid increase followed by a decay in yield stress due to the enzymatic action. The decay of the yield stress was more rapid for the initial loads , becoming slower as the hydrolysis progressed. The influence of fiber loading on the rheology of the slurry undergoing hydrolysis is discussed.